App: fix LinkBaseExtension::extensionGetSubObject()
App: fix LinkBaseExtension::extensionGetSubObject()

This fix obtain shape from a link to a sub element

Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master